Process for coating paper web



Patented Mar. 24, 1953 PROCESS FOR COATING PAPER WEB Joseph 'T. Loomer, Waterford, Conn, assignor to Robert Gair Company, Inc., New York, N. Y., a

, corporation of Delaware No Drawing. Application April 4, 1950,

Serial No. 154,001

6 Claims.

This invention relates to a coating process and more particularly to the coating of paper or paper board.

The process of the instant invention produces a coated paper board having a coating which has been rendered insoluble to water in the process of manufacture and does not require aging for the insolubilization to take place.

The instant process further provides for the coating of sheet materials, at the same time enabling the coated material to be finally given a treatment to enhance its printability, color, or other desirable characteristics.

The instant invention further resides in a process in which the compositions used do not give ofi fumes that would be objectionable or hazardous to the operators of the process.

Theinstant inventionfurther resides in the provision of a process for producing a coated sheet material having very little objectionable odor residue.

'The' instant process further provides for the hardening of a coating outwardly from the paper as the process continues.

The process of the instant invention further provides that the adhesive content of the coating needed for certain printing operations may be substantially reduced without sacrificing printability in the final product.

.1 The instant invention further resides in a process of coating sheet materials that-produces a coating having a substantially greater pigment content than heretofore without adversely-afiecting-the important characteristics thereof.

Although the novel features which are believed to be characteristic of this invention will be particularly pointed out in the claims appended hereto, the invention itself, as to its objects and advantages, the mode of its operation, and the manner of its organization may be better understood by referring to the followin description: Generally speaking, the machine coating of paper or paperboard usually comprises the formation of a fibrous web, the partial drying of that web, the subjecting of that web to the smoothing action of rolls or a breaker stack calender, the smoothing on the surface of the web of an aqueous mineral coating, the subsequent drying of that coating, the smoothing of the resultant product by means of l calenders, known as finishing calenders, the aforementioned sequences taking place in one continuous operation. 1 Of course, the order of these operations is varied to fit the circumstances.

" As normally practiced in industrial concerns,

2 the operation of the'coaters is usually as follows; The fibrous web is produced and partially dried. It is then subjected to the action of smoothing rolls or a calender known as a breaker stack. It is a usual practice to apply water'or aqueous solutions of starches, proteins, alginates, polyvinyl alcohol, carboxymethyl cellulose, hydroxyethyl cellulose, and other water soluble or water dispersible agents to the surface of the web at these calenders by means of a waterbox'and to condition the surface of the web prior to coating.

While still in a moist condition, the web is then passed into the coater where an aqueous suspension of mineral pigments with an adhesive is smoothed on the surface of the web. The web is subsequently dried bythe application of heat and then subjected to the action of finishing calenders. To enhance the action of these calenders, it has been necessary heretofore tooperat steam showers against the coated side of the web, instead of using the normal, waterbox treatment, These waterbox treatments normally produce a better quality product of more and desirable value than a product produced without operating the calenders as they were so intended.

Because of the short drying time and lack of water resistance in the present machine coating formulations, attempts to operate these finishing calenders with waterboxes have not resulted in success. Attempts to do so have usually resulted in the coating picking on these calender rolls with; loss of finish and in many cases-with complete interruption of the normal production by sheet breakage. By utilizing the methods of my invention it is possible to produce an economical machine coated paper or board which may be subjected to the normal applications of water, starches, proteins, methocel, hydroethylcellulose, calender colors, etc., applied over the coating without danger of milking or picking or interrupting production.

In the artof coating preparation, it is old in the art to add aldehydes to an aqueous mineral suspension with a protein adhesive to produce a coating on paper or board in which the coating becomes insoluble on aging, usually in one to three days or more. However, the addition of these aldehydes directly to the coating solution has usually resulted in an undersirable change in the batch life and the flow properties of the coatings. Moreover, these coatings do not waterproof sufficiently in the short length of drying space to permit operation of waterboxes on the finishing calenders in the continuous operation of machine coating; Furthermore, such an additionof the aldehyde to the coating mixture tends to "case harden the coated surface making drying difiicult.

In brief, my invention contemplates the application of an insolubilizing agent to the surface of the sheet to be coated and while still damp, the sheet is coated with an aqueous pigment suspension containing an adhesive, or an aqueous solution of the adhesive alone. The subsequent insolubilization takes place almost immediately after the application of heat in the short drying space between the coater and finishing calender, thus producing a coated board that may be subjected to the action of numerous solutions applied at the waterbox on the finishing calenders.

The insolubilizing or coagulating agent may be any of those materials, well known in the art, which will react with the adhesives to be mentioned hereinafter to insolubilize or harden them. In the particular application of the present invention, the aldehydes constitute the best coagulating agents, since they will so react with both protein and carbohydrate adhesives. In particular, we use'any aldehyde which does not give off fumes and which has no residual odor effect on the finished coated material. Examples of this type are methylal and ethylal. Another instance of this type of aldehyde and the one that we find desirable in the practice of this invention is glyoxal.

In practice we find it most convenient to introduce the glyoxal at the waterbox of the initial calender stack along with the desired calendering solution. Now, it is evident that the insolubilizing agent must not react with the calendering solution used to adversely affect the viscosity, etc., characteristics thereof. It has been found that, when aldehyde coagulating agents are used, dextrine-type starches and alginates are suitable.

The main coating ingredients, which are applied at the coater, comprise a water dispersion of adhesive, plus any pigments, colors, plasticizers, etc., that are needed for a particular application. The adhesives may be carbohydrate or protein, such as casein, soy bean protein, zein, polyvinyl alcohol, carboxymethyl cellulose, starch, or other water-dispersible adhesives capable of being insolubilized, but I prefer to use protein and more specifically casein. Of course, the concentration of the insolubilizing material is adjusted to bring about complete insolubilization of the particular adhesive used.

It can be seen, then, that in the preferred manner of carrying out my invention the sheet is coated with an aqueous solution of glyoxal Or an aqueous solution of glyoxal and dextrine starch, and passes'through the initial calendering stack.. Then, while the sheet is still damp with this mixture, it isgiven the main coating, which may be a protein adhesive and pigment, or a protein material alone.

This means that the web is carrying a coating of glyoxal and superimposed on this initial coating is a second coating which, as stated, may comprise a mineral pigment with a protein adhesive, or a protein solution alone. It is evident that that part of the second coating lying adjacent the paper web will absorb the highest concentration of glyoxal as deposited by the first coating and will, therefore, tend to harden sooner than the part which comprises the outer part. Now, the sheet is then exposed to heat, preferably by radiation, and, of course, the outer portions of the second coating, which portions 4 have the lowest concentration of glyoxal will receive the greatest amount of heat from the heat source. This means that these outer portions will harden as fast as the inner portions, despite their low concentration of the insolubilizing agent, since the chemical action of hardening will proceed so much faster because of the higher temperature in those portions of the coating. When the sheet reaches the finishing calender stacks, the coating has been insolubilized throughout, and can be subjected to any finishing and calenderizing solutions common to mill practice desired without danger of these solutions dissolving any portions of the coating. This, of course, means that I have avoided the case hardening of the coating where only the outer portions of the coating are hardened on reaching the finishing calender stack, so that a soft layer of material lies between the outer, hard portions of the coating and the paper, with resultant slippage between the two and other undesirable effects.

It is evident, also, that the use of selected aldehydes overcomes the danger of fumes in the mill, and also removes any possibility of an objectionable odor remaining with the sheet. This is particularly important where the sheet material is to be used in boxes for the dispersing of foods. At the same time, the practice of the process of my invention overcomes the disability inherent in using a mixture of the adhesive and the insolubilizing agent, which mixture changes viscosity rapidly as the adhesive coagulates, with the consequent difficulty of obtaining uniform work.

Since my process permits the application of a finishing solution over the insolubilized coating, it is possible to reduce the adhesive to pigment ratio in the mineral coating and yet present a superior printing surface. I-Ieretofore the printability of coatings has been governed by the adhesive to pigment ratio of the coatings. With my process, the finishing solutions employed act to reinforce the adhesive already in the coating as well as seal off the surface of the coating, thus providing a superior and more easily controlled printing surface. The advantages inherent in the use of the main coatings of the type just described are as follows:

The coatings possible by the practice of my invention are advantageous from the appearance of the finished product, and the high concentration of pigment to adhesive is cheaper and more opaque.

It is evident, also, that applying the insolubilizing agent in advance of the coating prevents absorption of the coating substance by the sheet material itself, particularly if, as has been suggested, a surfacing material, such as starch, be introduced onto the sheet with the coagulating agent.

While certain features of the invention have been shown and described and are pointed out in the annexed claims, it will be understood that various omissions, substitutions, and changes in the details of the invention specifically described may be made by those skilled in the art without departing from the spirit of the invention.

What I claim is:

1. A process for treating a paper web comprising the step of applying an aqueous solution of an aldehyde to the surface of the web, the step of passing the same through rolls to smooth the surface, the step of coating the web, while still damp with said solution, with an aqueous mineral suspension containing an adhesive which insolubilizes under reaction with an aldehyde, and the step of subjecting the web to the action of finishing calenders and at the same time treating the surface of the coating with a {finishing solution.

2. The process of coating a paper web comprising the steps of subjecting it to an aqueous solution of glyoxal and dextrine starch, calendering it, coating it while still damp with an aqueous solution of casein and pigment, drying the coating by means of radiant heat that is directed onto the exposed surface of the coating,

subjecting it to the action of a surface-finishing solution, and finish calendering the material.

3. In the manufacture of coated paper web the process which comprises the steps of treating said web with glyoxal mixed with dextrine starch in an aqueous solution, coating the treated material with an aqueous solution of a pigment and an adhesive which is insolubilized by glyoxal, heat-drying the coating, and subjecting it to the action of a finishing solution.

4. In a process of treating paper webs which comprises passing a freshly made paper web through a series of coat applying rolls, then, while the webis in damp condition applying a coating comprising a mineral material and a carbohydrate binder in an aqueous vehicle, then subjecting the web and coating to treatment with a finishing solution using the waterbox method of application, the improvement which resides in the step of adding an aldehyde solution to the web at a point substantially preceding the application of the said coating, said aldehyde insolubilizing said coating.

5. In the process of treating paper webs which comprises passing a freshly made paper web through a series of solution-applying and surface-smoothing rolls and while the web is in damp condition applying an opaque coating of high solids content comprising a mineral pigment and a protein adhesive in an aqueous vehicle, then subjecting the web and coating to heating and then to a treatment with a finishing solution using the waterbox method of application, the improvement which resides in the step of adding glyoxal to the web at a point substantially preceding the application of said coating, whereby the coating is completely and uniformly insolubilized by the time it reaches the point of waterbox treatment.

6. In the manufacture of coated paper, the process which comprises the steps of first treating the paper with an aldehyde mixed with a dextrine starch in an aqueous solution, then coating with a mixture of a pigment and an adhesive which is insolubilized by aldehydes, subjecting the surface of the adhesive coating to radiant heat, treating the resulting coated surface with a finishing solution, and subjecting it to a calendering action.

JOSEPH T. LO'OMER.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,123,152 Rivat July 5, 1938 2,177,580 Porter Oct. 24, 1939 2,263,576 Glenn Nov. 25, 1941 2,286,259 Cagle June 16, 1942 2,369,427 Bennett Feb. 13, 1945 2,414,858 Davidson Jan. 28, 1947 2,460,998 Nelson Feb. 8, 1949 2,515,340 Fisher et al July 18, 1950 

1. A PROCESS FOR TREATING A PAPER WEB COMPRISING THE STEP OF APPLYING AN AQUEOUS SOLUTION OF AN ALDEHYDE TO THE SURFACE OF THE WEB, THE STEP OF PASSING THE SAME THROUGH ROLLS TO SMOOTH THE SURFACE, THE STEP OF COATING THE WEB, WHILE STILL DAMP WITH SAID SOLUTION, WITH AN AQUEOUS MINERAL SUSPENSION CONTAINING AN ADHESIVE WHICH INSOLUBILIZES UNDER REACTION WITH AN ALDEHYDE, AND THE STEP OF SUBJECTING THE WEB TO THE ACTION OF FINISHING CALENDERS AND AT THE SAME TIME TREATING THE SURFACE OF THE COATING WITH A FINISHING SOLUTION. 